1. Field of the Invention
This invention relates to a recycle developing method used in an electrophotographic apparatus such as a copying machine or a printer.
2. Description of the Related Art
Generally, image formation in electrophotography is carried out by charging (main charging) the surface of a photosensitive material, imagewise exposing the charged material to form an electrostatic image on the surface of the photosensitive material, developing the image with a developer filled in a developing vessel to form a visualized toner image, transferring the toner image to a predetermined paper, and removing a toner remaining on the photosensitive material after the transferring by using a device such as a cleaning blade to complete one cycle of image forming step.
A typical developer is, for example, a two-component magnetic developer composed of an electroscopic toner made of a colored resin composition and a magnetic carrier. The developer is delivered to a developing zone in the form of a magnetic brush by means of a developer conveying sleeve provided in a developer vessel, sliding the magnetic brush with the electrostatic image on the photosensitive material, and adhering the toner to the electrostatic image to thereby perform development.
Recently, many recycle developing methods in which a toner removed and recovered by cleaning is again recirculated into a developing vessel and again used for development with an object of reutilizing toners have been proposed, and have been applied to actual electrophotographic apparatuses. This recycle developing method is applied to inexpensive low speed machines using organic photosensitive materials (OPC) in general.
In this recycle developing method, when a toner in the starting developer filled in the developing vessel is consumed and reaches a concentration of a predetermined level or below, a virgin toner is replenished from a toner feeding hopper, and a toner recovered by cleaning is also replenished.
However, in the above-mentioned recycle developing method, there is a problem in that the properties of a recovered toner to be used again after recovering by cleaning are different from the properties of a toner contained in the starting developer or those of a virgin toner supplied to a developing vessel.
For example, the surface of the toner is surface-treated with a treating agent such as silica or alumina so that its properties such as flowability may be held stably. However, the toner which is supplied for development is adhered to the surface of the photosensitive material and thereafter is recovered by cleaning, the surface treating agent is removed or embedded in the toner particles due to an external force of cleaning or to a force exerted after recovering in the step of conveying into the developing vessel, and the charged amount of the toner becomes low. Hence, as the recovered toner is replenished into the developing vessel, the charged amount of the toner in the developer becomes low, and inconveniences such as fogging or toner scattering occur.
In the developing vessel, a toner concentration sensor is provided so that the toner concentration (T/D) of a developer composed of a toner and a carrier may be controlled within a predetermined range. Controlling of this toner concentration is carried out by utilizing the variation of the toner concentration in the developer corresponding to the magnetic permeability of the developer. The magnetic permeability of the developer is detected by the toner concentration sensor and the toner is replenished into the developing vessel according to the output value of the sensor.
Changes in the properties of the developer by the replenishing of the recovered toner adversely affect the controlling of the toner concentration. For example, curve A in FIG. 3 shows the relation between the output (corresponding to the magnetic permeability of the developer) of the concentration sensor in the starting developer and the toner concentration (T/D). According to this curve, if the threshold value of ON-OFF of toner replenishing is set at a sensor output value 3 V, the toner is replenished into the developing vessel when the toner concentration becomes 3.5% or below. However, when the recovered toner is replenished into the developing vessel, the properties of the developer vary and the relation between the output of the concentration sensor and the toner concentration changes to, for example, curve B. Therefore, by the above-mentioned setting of the threshold value, it will become difficult to maintain the toner concentration at a predetermined level.